Giant aneurysms (measuring greater than 25 mm), are relatively rare, and are found mostly in the cavernous and paraclinoid segments, and in the internal carotid and the vertebrobasilar tree. They often present with a mass effect, intracerebral hemorrhage, cranial neuropathies and thromboembolism. [1],[2] Giant aneurysms not only have a poorer natural history but are also relatively difficult to cure completely both utilizing the endovascular as well as the surgical techniques. Curative treatment planning depends upon the size, site, presence of collateral flow, morphology of the aneurysm and the presence of a thrombus or calcification. [3] Cerebral angiography with balloon occlusion test is the standard procedure to evaluate for the presence of collateral supply from the communicating arteries. It has to be supplemented by other investigations like magnetic resonance imaging (MRI) and computed tomography (CT) to assess for hemorrhage, infarction, mass effect, perilesional edema and the presence of thrombus. [2]

With keen interest, we read this article "Endovascular management of giant aneurysms: An introspection" that includes 39 patients with a giant aneurysm. This study points out that untreated giant aneurysms have a higher morbidity and mortality compared to patients treated with the endovascular technique. [4] This is similar to the large study conducted by Winn et al. that showed a mortality rate of almost 67% due to rebleed and the chances of rupture at 3.5% per year in the first decade of follow up. [5]

Broadly, the treatment of these aneurysms is divided into 2 forms, deconstructive and reconstructive. The deconstructive therapy comprises of parent artery occlusion (PAO) either surgically or by the endovascular means using coils. PAO is planned only if there is adequate collateral flow. PAO with or without bypass is still the best option and is more likely to yield permanent results. [6],[7] However, these patients need a long-term follow-up to assess for the appearance of new aneurysms due to the resultant changes in hemodynamics and the ischemic complications. Recently, a study has revealed that external carotid-to-internal carotid (EC-IC) artery bypass of the occluded segment may prevent the ischemic complications that may potentially occur despite the presence of adequate collateralization proven by a prior balloon occlusion test (BOT). Hence, an EC-IC bypass may help in rescuing unanticipated hemodynamic insufficiency. [7] Endovascular PAO may occasionally fail due to its inability to block blood flow to the aneurysmal neck that may persist via the vasa vasorum that may be present within the adventitia of neck of the aneurysm beyond the occluded arterial segment. [8]

In reconstructive endovascular treatment, the parent artery is preserved. There are two forms of repair viz., 1) endosaccular, which is aimed at filling the sac that leads to aneurysmal thrombosis; and, 2) endoluminal, which is aimed at restoring the parent vessel wall defects. [9]

The endosaccular repair is done using various types of retrievable and detachable platinum coils. These coils are of various sizes, shapes and softness. Newer hydrocoils that can swell up after 6-8 min on contact with blood can provide a more dense packing of the aneurysm. Bioactive coils induce more inflammation and cause a faster thrombosis of the aneurysm. [10] Biocompatible ethylene-vinyl alcohol copolymer, Onyx HD 500 has been used in wide-necked aneurysms (where coiling is difficult) with good results. Currently its use is limited to large or giant, wide-necked aneurysms situated proximal to the posterior communicating arteries. [11] For wide-necked aneurysms, a remodeling technique using either a balloon (Hyperglide, Hyperform, Eclipse, Sceptre) or stent not only prevents the coil from prolapsing into the parent vessel but also helps in achieving better packing of the aneurysmal sac. [9]

Closed cell design stents (Leo and Enterprise) are preferred over the open cell design ones as these are retrievable even after 90% deployment. The open cell design stents like the Neuroform stents are difficult to deploy and have chances of strut prolapse within the curved vessels like the cavernous ICA. Aneurysms located at the middle cerebral artery bifurcation or at the basilar top can be treated by deploying 2 stents in a "Y" configuration. Telescopic stents (i.e., placing one stent inside the other) had achieved good results prior to the availability of specialized flow diversion stents. [12],[13] Stent-assisted procedures also have their disadvantages like branch artery or perforator vessel occlusion, in-stent thrombosis shortly after antiplatelet medications are discontinued, the need for long-term antiplatelet therapy, catastrophic bleeding complications and, difficulties encountered in deploying the stent in a distal aneurysm due to stiffness of the stent. [9] Mawad et al. achieved high cure rates in the treatment of giant aneurysms with stent-assisted coiling (SAC) with no re-canalization at 6 months in 81% cases. [14] Contrary to this, Mu et al. found 67% failure rate in SAC of large or giant aneurysms. [15]

Flow diverters (FD) are the latest features in the series of advancements in the reconstructive endovascular treatment of aneurysms. Various devices like silk flow diverters, Flow Re-direction Endoluminal Device (FRED) and Pipeline Embolization Device (PED) have been tried in the treatment of giant aneurysms. [9],[16],[17] FDs cause endoluminal reconstruction, change the flow dynamics in and out of the aneurysm, and subsequently induce thrombosis within the aneurysmal sac. Later on, neointimal proliferation occurs which eventually covers the stent leading to restructuring of the parent artery. This prevents parent artery sacrifice and also spares the perforator arteries. Subsequently, the aneurysm gets thrombosed and the sac shrinks around the stent relieving the mass effect. Although stasis may be seen immediately but complete resolution of the aneurysm can take a longer time. [18] Lanozi et al. showed a higher rate of complete obliteration with FDs compared to the conventional coiling. [19] However, no similar studies are available for endovascular treatment of giant aneurysms. The major advantage of FD is the non-requirement for catheterization of the delicate aneurysmal sac thus reducing the risk of rupture. In complex aneurysms, it is a single step procedure unlike the conventional method which would need numerous coil deployments and repeated repositioning of the microcatheter. [16] Like other stents, FDs carry the risk of occlusion, thrombo-embolism and failure. [9] Most of the failures with FD have been noted with giant aneurysms. Darsaut et al. reported persistent filling of the aneurysm due to a deformed FD construct. [20] Animal model studies have shown that even very low porosity FDs failed to occlude the aneurysm in the cases of giant aneurysms arising from a curved bifurcation of vessels and from its side walls. [21] Newer specialized flow disruptors are expected in the near future like the "Sphere" FDs that are specially designed for bifurcation aneurysms. [22]

Indeed the treatment of giant aneurysms is challenging. With rapidly advancing developments and increasingly sophisticated devices being made readily available in the armamentarium of endovascular procedures, we are hopeful of achieving better and more complete cure rates with fewer complications. A randomized control trial is being carried out to compare the aneurysmal cure rate and its recurrence rate after traditional coiling and flow diversion. [23] FDs show promise as the simplest and probably the best endovascular modalities available at present; however, robust long-term results are still awaited to support their use in giant aneurysms.

Finally, endovascular parent artery occlusion (PAO) forms the best and the most durable treatment of giant aneurysms. On the long-term basis, however, new aneurysms may form due to changes in the vessel hemodynamics utilizing this technique. EC-IC bypass and PAO may provide durable results and prevent new aneurysm formation but EC-IC anastomosis is successful in only about 90% cases even in the best of hands. Placement of flow diverters may be the best and the simplest treatment option available at present; a long-term follow-up is still awaited.